`_edf` submodule restructuring

[teonbrooks]

I was browsing the Credit page and noticed that it lists a private module among the others. Two thoughts occurred:

1. maybe we should filter private submodules from the list. I noticed the _fiff submodule isn't there so maybe the filter just needs to be generalized.
2. Maybe this submodule with its reading function for filelike object could be added to the edf module since it already has content for edf and bdf reading functions are.

[larsoner]

1. We filter some private submodules already, so yeah we should add filters. Or really, we should assert that there are no _-prefixed modules being documented on that page, yelling that the code needs to be updated to lump those into the most appropriate public module. (For example, I think -- but haven't checked -- that changes to mne._fiff get lumped into mne.io by the credit code because they are most closely / extensively related to that submodule.)
2. That submodule is indeed in a weird place, we should move it. Agreed mne/io/edf somewhere would make sense.

[Aniketsy]

we should assert that there are no _-prefixed modules being documented on that page

@larsoner I think we can go ahead with this, I’d be happy to look into this and open a PR if that sounds good.

[larsoner]

Sure go ahead @Aniketsy